1. Field of the Invention
The invention relates to perforating guns employed for perforating the casing and adjoining production formation of a subterranean well, and to a method for operating said gun so that the vertically spaced charges disposed in the gun are fired from the bottommost charge upwardly to the uppermost charge.
2. History of the Prior Art
The utilization of a plurality of vertically and angularly spaced shaped charges to effect the perforation of a subterranean well represents the modern approach to achieving perforation of the well casing and the adjoining production formation. In recent years, it has become desirable to mount the shaped charges of the perforating gun within or upon a tubular housing, which in turn is run into the well on the bottom of a tubular string, which may comprise the production string. To provide additional protection of the shaped charges and the primer cord for actuating such shaped charges from the potential adverse effects of contact with well fluids, it is customary to effect a sealed connection of all components of the perforating gun housing containing the shaped charges in order that the interior of the housing is isolated from well fluids. As is well known to those skilled in the art, even though the most carefully assembled units may, after being run into the well, and particularly a well having substantial deviations from the true vertical, experience stresses which result in the creation of leakage paths for well fluids to enter the perforating gun housing. Multisection guns having an overall length on the order of 100 to 2,000 feet are particularly subject to inadvertent leakage. Since the shaped charges are normally sealed within a container, and the primer cord for activating such shaped charges is encased within a thermoplastic tube, the existence of well fluids within the perforating gun housing will not necessarily prevent the detonation of the primer cord and the subsequent detonation of the shaped charges. The presence of fluids within the housing during the detonation of the shaped charges results in a very substantial hydraulic expansion force being exerted on the wall of the perforating gun housing. In some instances, the expansion is sufficient to bulge the housing into tight engagement with the casing and thus effectively prevent the removal of the perforating gun from the well.
It follows that there is a definite need in the subterranean well industry for the method and apparatus for perforating the well casing and the adjoining production formation which will not fire the shaped charges when the lower portions of the perforating gun housing are partially filled with well fluids but, in the absence of any substantial amount of well fluids, will achieve the firing of all of the shaped charges mounted within the perforating gun housing.